The detailed calorimetric analysis of the thermodynamics of the allosteric enzyme aspartate transcarbamylase will be continued. Both the native enzyme and chemically modified or mutant enzymes will be used to investigate the energetics of the interactions between the subunits and of the binding of substrates, inhibitors, and activators. The purpose of these measurements is to: (a) establish thermodynamic criteria against which models of the catalytic and regulatory mechanisms may be tested, (b) evaluate the magnitude of the various conformational changes which the enzyme undergoes and (c) elucidate the nature of the forces involved in binding small molecules to the enzyme and in subunit interactions. An analysis of the thermodynamic properties of erythrocyte membranes will be begun. Flow microcalorimetry will be used to characterize the binding of plant lectins, prostaglandins, glyceraldehyde-3-phosphate dehydrogenase, aldolase and spectrin. These measurements will contribute to our understanding of lipid-protein interactions and cooperative structural changes in membranes. BIBLIOGRAPHIC REFERENCE: Calorimetric Analysis of Aspartate Transcarbamylase from Eschericia coli: Binding of Cytosine 5'-Triphosphate. Allewell, N.M., J. Friedland and K. Niekamp. Biochemistry (1975) 14:224.